because developers are complaining that they have to pay a commission to Google and Apple and they have to do that because Google and Apple have a duopoly in the mobile os market. If consumers were spread amongst a greater number of OSes then developers could more easily choose to ignore Android and iOS and just make their app compatable with whatever other OSes are out there, which in turn puts competitive pressure on Google and Apple to do more to woo those developers.Why is that what we actually need? As I have tried to make clear, the EU can use regulations to create more competitive markets when there are large dominant players that are abusing their position. The EU cannot create new players in a market by fiat declaration, no one can. The EU can only lower barriers caused by bad corporate behaviour they cannot lower barriers that amount to things like "time has passed".
Another thing that isn't clear is why you want more OS players, the argument you're making seems to be that if we had more OS choice then the App Store lock in and abuse would be just fine and nothing to worry about. However there is nothing to say that the market couldn't end up just being iOS - 40% Android - 40% then a bunch of companies with 5% each, then App Store lock in and abuse would be just as big of a problem as it is today.
as it stands, the DMA will not do anything to loosen Google and apples duopoly on either the OS market or the App Store market. We need more OSes to achieve that.